1. Field of the Invention
The present invention relates to vehicular electronic apparatuses, and more particularly to a vehicular electronic apparatus in which interference of a received a broadcast wave is suppressed.
2. Description of the Related Art
In the receiver, various noises are to be mixed therein depending upon a state of radio wave propagation, causing problems with reception. Particularly, in an FM receiver mounted on a vehicle, there is an increased possibility of interference by electromagnetic waves caused by different mounted electronic apparatuses with increasing kinds and numbers of electronic apparatuses mounted on the vehicle. In addition, the receiver is also susceptible to the noise caused by fluctuations in the received electric field due to the vehicle traveling, reflection upon buildings, and so on.
With respect to the EMI (Electro Magnetic Interference) caused by the electromagnetic wave generated by the electronic apparatus mounted on a vehicle, countermeasures are taken to suppress interference wave radiation by attaching, on the receiver side, a noise suppressing unit such as a filter or a shield and on the vehicle-mounted electronic apparatus side by selecting a CPU for use in the electronic apparatus, using an appropriate printed-board pattern, or adding countermeasure parts such as a shield, for example.
As described above, the vehicular electronic apparatus generates various interfering waves (unwanted electromagnetic waves). Particularly, recently the number and kind of vehicular electronic apparatuses in which microcomputers are mounted has increased dramatically. This, in turn, increases the noise caused by a crystal oscillator or the like in each apparatus that supplies a clock to the microcomputer, which causes disturbance on the receiving unit of the FM receiver.
In the meanwhile, where a signal is applied to a circuit, such as an amplifier, constituting a receiver, the non-linearity in the amplifier input-output characteristic, if present, causes distortion in its output.
For example, on the vehicular receiver for receiving a broadcast wave of the FM receiver or the like, electromagnetic waves which have frequencies other than that of the desired frequency (such as the foregoing interference waves) are simultaneously received with the electromagnetic wave of the desired frequency. Consequently, a plurality of signals that have different frequencies are simultaneously applied to the amplifier of a circuit close to the antenna.
In this manner, in case two signals different in frequency are applied simultaneously to the amplifier, the amplifier generates a signal having a new frequency component besides the outputs responsive to the respective frequency components of the two applied signals.
The amplifier response characteristic, when two signals are applied to the amplifier, is called the “two-signal characteristic” and is expressed as follows.
Assuming t is time, the amplifier input is represented as x(t) and the output as y(t). If the amplifier has a non-linearity in its input-output characteristic, the output y(t) can be approximated as a polynomial of t, as in Equation (1).y(t)=ax(t)+bx(t)2+cx(t)3+ . . .   (1)
Subsequently, to this amplifier is added by an input signal {x1(t), x2(t)} represented by Equation (2). Equation (2) expresses two sinusoidal waves different in frequencies (f1 and f2). The amplifier output at this time can be determined by substituting Equation (2) into Equation (1). This result is given as Equation (3), which is expanded into Equation (4). Incidentally, because the coefficients in the expansion are complex and unnecessary for this illustration, they have been omitted for convenience.x(1)=A cos (2πf1t)
x(2)=B cos (2πf2t)  (2)                                                                        y                ⁡                                  (                  t                  )                                            =                            ⁢                                                a                  ⁢                                      {                                                                  Acos                        ⁡                                                  (                                                      2                            ⁢                            π                            ⁢                                                                                                                  ⁢                            f1t                                                    )                                                                    +                                              B                        ⁢                                                                                                  ⁢                                                  cos                          ⁡                                                      (                                                          2                              ⁢                              π                              ⁢                                                                                                                          ⁢                              f2t                                                        )                                                                                                                }                                                  +                                                                                                      ⁢                                                b                  ⁢                                                            {                                                                        A                          ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                                                              2                                ⁢                                π                                ⁢                                                                                                                                  ⁢                                f1t                                                            )                                                                                                      +                                                  B                          ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                                                              2                                ⁢                                π                                ⁢                                                                                                                                  ⁢                                f2t                                                            )                                                                                                                          }                                        2                                                  +                                                                                                      ⁢                                                c                  ⁢                                                            {                                                                        A                          ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                                                              2                                ⁢                                π                                ⁢                                                                                                                                  ⁢                                f1t                                                            )                                                                                                      +                                                  B                          ⁢                                                                                                          ⁢                                                      cos                            ⁡                                                          (                                                              2                                ⁢                                π                                ⁢                                                                                                                                  ⁢                                f2t                                                            )                                                                                                                          }                                        3                                                  +                …                                                                        (        3        )                                                                                    y                ⁡                                  (                  t                  )                                            =                            ⁢                              k0                +                                  k1                  ⁢                                                                          ⁢                                      cos                    ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                        f1t                                            )                                                                      +                                  k2                  ⁢                                                                          ⁢                                      cos                    ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                        f2t                                            )                                                                      +                                  k3                  ⁢                                                                          ⁢                  cos                  ⁢                                      {                                          2                      ⁢                                                                                          ⁢                                              π                        ⁡                                                  (                                                      2                            ⁢                                                                                                                  ⁢                            f1                                                    )                                                                    ⁢                      t                                        }                                                  +                                                                                                      ⁢                                                k4cos                  ⁢                                      {                                          2                      ⁢                                              π                        ⁡                                                  (                                                      2                            ⁢                            f2                                                    )                                                                    ⁢                      t                                        }                                                  +                                  k5                  ⁢                                                                          ⁢                  cos                  ⁢                                      {                                          2                      ⁢                                              π                        ⁡                                                  (                                                      f1                            +                            f2                                                    )                                                                    ⁢                      t                                        }                                                  +                                  k6cos                  ⁢                                      {                                          2                      ⁢                                              π                        ⁡                                                  (                                                      f2                            -                            f1                                                    )                                                                    ⁢                      t                                        }                                                  +                                                                                                      ⁢                                                k7                  ⁢                                                                          ⁢                  cos                  ⁢                                      {                                          2                      ⁢                                              π                        ⁡                                                  (                                                      3                            ⁢                            f1                                                    )                                                                    ⁢                      t                                        }                                                  +                                  k8                  ⁢                                                                          ⁢                  cos                  ⁢                                      {                                          2                      ⁢                                              π                        ⁡                                                  (                                                      3                            ⁢                            f2                                                    )                                                                    ⁢                      t                                        }                                                  +                                  k9                  ⁢                                                                          ⁢                  cos                  ⁢                                      {                                                                  2                        ⁢                                                  π                          ⁡                                                      (                                                                                          2                                ⁢                                f1                                                            -                              f2                                                        )                                                                          ⁢                        t                                            +                                                                                                                                                            ⁢                                                k10                  ⁢                                                                          ⁢                  cos                  ⁢                                      {                                          2                      ⁢                                              π                        ⁡                                                  (                                                                                    2                              ⁢                              f2                                                        -                            f1                                                    )                                                                    ⁢                      t                                        }                                                  +                                  k11                  ⁢                                                                          ⁢                  cos                  ⁢                                      {                                                                  2                        ⁢                                                  π                          ⁡                                                      (                                                                                          2                                ⁢                                f1                                                            +                              f2                                                        )                                                                          ⁢                        t                                            +                                                                                                                                                            ⁢                                                k12                  ⁢                                                                          ⁢                  cos                  ⁢                                      {                                          2                      ⁢                                              π                        ⁡                                                  (                                                                                    2                              ⁢                              f2                                                        +                            f1                                                    )                                                                    ⁢                      t                                        }                                                  +                …                                                                        (        4        )            
Subsequently, the terms of Equation (4) are explained.
The first term is a term having only the coefficient. This term is constant and does not change in time. Namely, it signifies that direct current is included in the output.
The second term represents an output of the basic wave produced by the input. If it is assumed that the amplifier is completely linear, only the coefficient of this term is finite while the other coefficients are zero. In other words, the components other than this term are all newly generated components by the amplifier non-linearity.
The third term has respective second harmonic components of the two input signals. The higher harmonic component is also generated a single signal is applied to the amplifier.
The fourth term is a component of a sum of and difference between two input signal frequencies. By utilizing this component, a heterodyne frequency converter circuit can be configured.
The component of the third and fourth term is generated by the secondary coefficient part of Equation (1). Accordingly, to extract or frequency-convert a second harmonic, the circuit constant is determined such that the relationship of input and output is approximated by a secondary function.
The fifth term is a third harmonic component. The sixth and seventh terms are each a component of a sum of and difference between one second harmonic and the other frequency. These terms resemble the fourth term. Incidentally, the fifth, sixth and seventh term have tertiary coefficients.
Generally, the frequency component “mf1±nf2” (m, n=1, 2, 3 . . . ) caused by the non-linearity of a circuit, such as amplifier, is called “intermodulation product”.
The two input signal frequencies f1 and f2 have a difference (f1−f2) of 1 kHz. This means that the component in the latter part of the fourth term has a frequency of 1 kHz, e.g. is heard as noise by the FM-broadcast listener.
Incidentally, although the coefficient notation was omitted in the foregoing Equation (4), the coefficients of the second term are representative of a basic wave component (f1) includes an amplitude value A of one input signal and an amplitude value B (AB2) of the other input signal B. This means that the amplitude of one signal is influenced by the amplitude of the other signal. Consequently, changing the amplitude of the other signal causes a variation in the amplitude of one signal. This phenomenon is called “cross modulation distortion”, which is heard as noise by the listener, similarly to the above.
Where an FM broadcast or the like is received by using a vehicular receiver, noise during traveling cannot be heard due to masking by engine sound. However, in the case of reception while the vehicle is stopped, distortion based, for example, on the foregoing intermodulation is readily heard as noise.
As noted before, countermeasures are taken to suppress interference wave radiation by attaching, on the receiver side, a noise suppressing unit, such as a filter or a shield while, on the vehicle-mounted electronic apparatus side, by selecting a CPU for use in the electronic apparatus, using an appropriate printed-board pattern or adding countermeasure parts such as a shield, for example.
However, the effects of suppressing the interference wave radiation are not readily predictable. In many cases, the final decision of what countermeasures must be provided is impossible without providing them on each actual vehicle. For this reason, the length of time required for design or assessment is significant, correspondingly increasing the manufacture cost.
The present invention has been made in view of these problems, and it is an object thereof to provide a vehicular electronic apparatus capable of effectively suppressing the interference with broadcast wave reception resulting from intermodulation.